Climb performances

[alpinehelicopter]

In some Flight Manual I noticed, that helicopters (like the EC120 or Lama) climb better in higher altitudes than at sea level:

at 2000ft ISA: 600ft/min
at 3000ft ISA: 700ft/min

I expected contrary because of the decreasing density. Maybe an aerodynamic reason....?

Does someone know the answer?

Thanks for answering!